Mississauga Astronomical Society

Fortyninth Meeting

Speaker’s’ Night

 

Day:                October 21, 2005

 

Speaker:     Professor Jimmy Law 

                  

                              

 

Neutrinos, Sunshine and SNO

 

Professor Jimmy Law of the University of Guelph spoke about the Sudbury Neutrino Observatory (SNO).  The Universities of British Columbia, Queens, Carleton, Guelph, Laurentian and TRIUMPH laboratories in B.C. together with institutions in the USA, UK and Portugal make up the SNO collaboration.  SNO was built in order to detect neutrinos from the Sun, to solve the solar neutrino problem and to detect other neutrinos.   

 

Professor Law outlined the history of the neutrino.  The “little neutral one” was named by Enrico Fermi after W. Pauli in 1933 postulated its existence to explain and allow conservation of energy and angular momentum in beta nuclear decay, following previous work done work by  Van Bayer et al, Chadwick and others.  After further work by many scientists, Cowan and Reiner detected the anti-electric neutrino in 1956.  This was followed by Lederman et al. who found the muon neutrino in 1962.  In 1964, Davis and his group set up a detector with 100,000 gallons of cleaning fluid in Homestead mine and gathered data.  In 1968 they reported that the observed solar neutrino flux was only 1/3 of predicted.  Explaining this result became the solar neutrino problem. A large tank is required because the cross section for interaction of the neutrino is extremely small.  With SNO, 8,000 interactions per year are expected.  Neutrinos come in 3 flavours: electron as observed from beta decay, muon as from pion decay, and tau as first observed in Fermi lab in 2000. 

 

In 1936, Bethe wrote equations for nuclear reactions that cause the sun to shine. Several of these show the production of neutrinos.  Experiments consistently demonstrated about 1/3 the number of neutrinos predicted.  Either the solar models were incomplete or incorrect, or the neutrinos undergo flavour oscillations. 

 

Then, along came SNO. Initially, the  observatory  was to look for proton decay but when this turned out to be impractical, Herb Chen of UC at Irvine pointed out the advantage of heavy water, readily available from the CANDU reactor, as a neutrino detector.  To get away from interference and to decrease operating costs, it was necessary to go underground to a working mine.  INCO in Sudbury was chosen.   A 12 foot square elevator descends to 2073 underground.  Everything brought in must fit into this space.  The detector is a 12 meter acrylic globe with 9500 photomultiplier tubes.  As a neutrino from the Sun hits an electron in the heavy water an elastic collision takes place causing the electron to move faster than light in that medium and it emits a cone of Cherenkov radiation that is detected by the tubes.  Professor Law explained that the heavy water at SNO is able to detect different neutrinos than can be detected by other sites such at Kakiomande in Japan.  He also explained the appearance of a neutrino event at SNO.

In phase I, a two year run was necessary to acquire a year of data and the results were released in 2002.   Phase II was run with salt because of the better efficiency using chlorine atoms.  A two year run is necessary to obtain a year of data because things (such as salt) have to be put into the detector, calibrations must be done, interference sources identified, and interruptions such as strikes and power outages endured.  Once raw data is obtained, spurious decays have to be eliminated, photons from the outside cut out, flash noise taken into account, etc.  

 

SNO was able to finally solve the solar neutrino problem by demonstrating that 65 percent of the electron neutrinos change to muon neutrinos – neutrinos oscillate and the smoking gun was found!  

 

Now, what do we do with this answer?  The universe contains a lot of missing mass and the neutrino mass cannot explain this.  Is there also a day-night effect considering that solar neutrinos may have to reach SNO through the Earth?  The answer is NO – there is no difference.  Is there a periodic variation in solar flux?  Again, the answer is NO.  We can, however, see variation with the Earth’s orbit, there being a greater flux when Earth is closer to the Sun.  

 

What’s next?  We need to confirm the salt (NaCl) results.  This is currently being done using neutral current detectors with the data completion planned for December 31, 2006.  After this, SNO might be dismantled and the D2O extracted  A new use for SNO could be other experiments such as studying whether deutrons can decay.  The underground facility can be expanded to allow several experiments simultaneously, one advantage being that the muon intensity in Sudbury is very low compared to other neutrino detectors.  Three types of proposals for future experiments for the new lab are looking for double beta decay, solar and geo neutrinos, dark matter (missing mass) studies. 

 

Finally, Professor Law finished his presentation with a movie showing the underground facility in the underground mine at the SNO.

 

 

Submitted by Chris Malicki, Secretary  Chris Malicki, Secretary                               back to M.A.S. meeting reports page
Mississauga Astronomical Society